1. Field of the Invention
This invention relates to magnetic heat treatment of steel, particularly to modification of microstructures of steel undergoing phase transformation in production. This is done by carrying out heat treatment with exposure to a specific magnetic field, followed by advantageous improvement of mechanical properties, as will be further discussed hereinafter.
2. Description of the Related Art
When steel is produced under usual production heating temperatures of 1200.degree. to 1300.degree. C., finish hot rolling temperatures of 800.degree. to 900.degree. C. and coiling temperatures of 400.degree. to 650.degree. C., its resulting microstructure usually comprises a mixed microstructure of phases of pearlite, bainite and martensite, with a ferrite phase and an austenite phase. It has long been known that the fine structure of this mixed microstructure depends on phase transformation which is brought about by cooling from high temperatures to low temperatures.
It is known according to an empirical equation of Hall-Petch that the finer the grains are, the greater the strength, ductility, toughness and fatigue strength of the steel. Accordingly, the constitution of the mixed microstructure is controlled by combination of composition, cooling rate and processing deformation of steel, and the microstructure is so refined that the prescribed characteristics can be obtained.
However, methods in which patterns of temperatures and processing are devised for every steel composition have reached their limits. Completely new methods are required.
A method in which a magnetic field is applied as an external field has been considered for the purpose. Sadovsky et all Fiz. Metal. Metalloved., vol. 12 (1961) p. 302! of Russia have made it clear by experiments that applying a magnetic field to steel elevates the temperature of martensite transformation and increases the martensite amount. It has been explained that in steel having an ordinary composition, the ferrite and martensite phases are ferromagnetic while the austenite phase is paramagnetic; therefore thermodynamic free energy of the former phase is reduced by magnetic energy. It is believed that, as a result, the driving force for transformation is increased by the magnetic field, so that the transformation temperature goes up. Recent researches have made it clear that the effect exerted on the transformation starting temperature includes high magnetic field susceptibility and forced volume magnetostriction, as well as the effect exerted by magnetostatic energy (T. Kakeshita et al, Japan Metallurgy Association Report 32 (1993), p. 591).
Motivation induced by these theoretical researches has led to researches relating to changing microstructure by applying a magnetic field to industrial common carbon steel Pustovoit et all, Metalloved. Term. Obrab. Met. (1979) 22). However, distinct effects are not necessarily obtained because the applied magnetic field is as small as 1 T (T designates the unit tesla which represents strength of magnetic field) and details on how to apply the magnetic field were not strictly investigated. The effect of magnetic field exerted on transformation has not yet been applied to the production of steel.
An object of the present invention is to provide magnetic heat treatment of steel by increasing the driving force for transformation, to refine the microstructure, and to improving the mechanical properties of the steel.